Demand for non-volatile memory (NVM) devices, including embedded NVM in other microelectronics and IC devices, has grown rapidly in recent years due to the expansion of digital computing and processing beyond desktop computer systems to include a broader array of consumer electronic, communications, automotive and industrial products. These products include mobile phones, still and video digital cameras, personal digital assistants (PDAs), portable computers, portable digital music players, digital video recorders, set-top boxes, communication routers and switches, digital televisions and other electronic systems. Each of these products typically requires one or more non-volatile memory device(s) to store data, such as the product's operating system and may also require data storage capabilities. The flash memory market, which in 2004 was the largest segment of the non-volatile semiconductor memory market, has traditionally been divided into four segments: code flash, data flash, embedded flash and serial flash.
Historically, the most widely-used technology for non-volatile semiconductor memory devices is floating gate technology, which was developed in the late 1960s and has been the prevalent technology for non-volatile semiconductor memory devices since then. A floating gate device is a variation of a standard metal oxide semiconductor (MOS) field effect transistor (FET) in that it has an additional electrically isolated “floating gate,” made of a conductive material. A floating gate device stores information by holding electrical charge within the floating gate. Adding or removing charge from the floating gate changes the threshold voltage (Vt) of the cell thereby defining whether the memory cell is in a programmed or erased state—representing a binary “1” or a binary “0” (memory cell states which may also be referred to herein as logic “1” and logic “0”), respectively or, conversely, binary or logic “0” and binary or logic “1”, respectively (the definition of the erase and program states as binary or logic “1” and binary or logic “0” being somewhat arbitrary, and generally at a designer's/manufacturer's discretion).
NROM technology effectively doubles the storage capacity of each memory cell by enabling the storage of two physically distinct and independent charges, each representing one bit of information, within a single memory cell. This significantly reduces the amount of silicon wafer required for each non-volatile memory device, resulting in a significant cost reduction to semiconductor manufacturers. Further advances in NROM and related ONO technology increase storage capacity to more than two bits (binary digits) per cell by better control and/or characterization of trapped charge.
Non-volatile memory devices based on NROM or other ONO (such as SONOS) technology contain a trapping nitride layer which stores a charge, instead of a floating gate suspended above the cell. The nitride layer is usually surrounded by two insulating silicon dioxide layers (oxide). Where applicable, descriptions involving NROM are intended specifically to include related oxide-nitride technologies, including SONOS (Silicon-Oxide-Nitride-Oxide-Silicon), MNOS (Metal-Nitride-Oxide-Silicon), MONOS (Metal-Oxide-Nitride-Oxide-Silicon) and the like used for NVM devices. Further description of NROM and related technologies may be found at “Non Volatile Memory Technology”, 2005 published by Saifun Semiconductor and materials presented at and through http://siliconnexus.com, “Design Considerations in Scaled SONOS Nonvolatile Memory Devices” found at: http://klabs.org/richcontent/MemoryContent/nvmt_symp/nvmts—2000/presentations/bu_white_sonos_lehigh_univ.pdf, “SONOS Nonvolatile Semiconductor Memories for Space and Military Applications” found at: http://klabs.org/richcontent/MemoryContent/nvmt_symp/nvmts—2000/papers/adams_d.pdf, “Philips Research-Technologies-Embedded Nonvolatile Memories” found at: http://research.philips.com/technologies/ics/nvmemories/index.html, and “Semiconductor Memory: Non-Volatile Memory (NVM)” found at: http://ece.nus.edu.sg/stfpage/elezhucx/mvweb/NVM.pdf, all of which are incorporated by reference herein in their entirety.
Commonly-owned patents disclose structure and operation of NROM and related ONO memory cells. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,768,192 and 6,011,725, 6,649,972 and 6,552,387.
Commonly-owned patents disclose architectural aspects of an NROM and related ONO array, (some of which have application to other types of NVM array) such as segmentation of the array to handle disruption in its operation, and symmetric architecture and non-symmetric architecture for specific products, as well as the use of NROM and other NVM array(s) related to a virtual ground array. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,963,465, 6,285,574 and 6,633,496.
Commonly-owned patents also disclose additional aspects at the architecture level, including peripheral circuits that may be used to control an NROM array or the like. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,233,180, and 6,448,750.
Commonly-owned patents also disclose several methods of operation of NROM and similar arrays, such as algorithms related to programming, erasing, and/or reading such arrays. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,215,148, 6,292,394 and 6,477,084.
Commonly-owned patents also disclose manufacturing processes, such as the process of forming a thin nitride layer that traps hot electrons as they are injected into the nitride layer. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,966,603, 6,030,871, 6,133,095 and 6,583,007.
Commonly-owned patents also disclose algorithms and methods of operation for each segment or technological application, such as: fast programming methodologies in all flash memory segments, with particular focus on the data flash segment, smart programming algorithms in the code flash and EEPROM segments, and a single device containing a combination of data flash, code flash and/or EEPROM. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,954,393 and 6,967,896.
The Field Effect Transistor
The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. Generally, a transistor has three terminals, and a voltage applied to a specific one of the terminals controls current flowing between the other two terminals.
The terminals of a field effect transistor (FET) are commonly named source, gate and drain. In the FET a small amount of voltage is applied to the gate in order to control current flowing between the source and drain. In FETs the main current appears in a narrow conducting channel formed near (usually primarily under) the gate. This channel connects electrons from the source terminal to the drain terminal. The channel conductivity can be altered by varying the voltage applied to the gate terminal, enlarging or constricting the channel and thereby controlling the current flowing between the source and the drain.
FIG. 1 illustrates a FET 100 comprising a p-type substrate, and two spaced-apart n-type diffusion areas—one of which will serve as the “source”, the other of which will serve as the “drain” of the transistor. The space between the two diffusion areas is the “channel”. A thin dielectric layer is disposed over the substrate in the neighborhood of the channel, and a “gate” structure is disposed over the dielectric layer atop the channel. (The dielectric under the gate is also commonly referred to as “gate oxide” or “gate dielectric”.) Electrical connections (not shown) may be made to the source, the drain, and the gate. The substrate may be grounded.
Generally, when there is no voltage on the gate, there is no electrical conduction (connection) between the source and the drain. As voltage (of the correct polarity) is applied to the gate, there is a “field effect” in the channel between the source and the drain, and current can flow between the source and the drain, and can be controlled by the voltage applied to the gate. In this manner, a small signal (gate voltage) can control a relatively large signal (current flow between the source and the drain).
The Floating Gate Transistor
A floating gate transistor is generally a transistor structure, broadly based on the FET, as described hereinabove. As illustrated in FIG. 2, the floating gate transistor 200 has a source and a drain, but rather than having only one gate, it has two gates which are called control gate (CG) and floating gate (FG). It is this arrangement of control gate and floating gate which enables the floating gate transistor to function as a memory cell, as described hereinbelow.
The floating gate is disposed over tunnel oxide (comparable to the gate oxide of the FET). The floating gate is a conductor, the tunnel oxide is an insulator (dielectric material). Another layer of oxide (interpoly oxide, also a dielectric material) separates the floating gate from the control gate.
Since the floating gate is a conductor, and is surrounded by dielectric material, it can store a charge. Electrons can move around freely within the conductive material of the floating gate (which comports with the basic definition of a “conductor”).
Since the floating gate can store a charge, it can exert a field effect on the channel region between the source and the drain, in a manner similar to how a normal FET works, as described hereinabove. Mechanisms for storing charges on the floating gate structure, as well as removing charges from the floating gate are described hereinbelow.
Generally, if a charge is stored on the floating gate, this represents a binary “1”. If no charge is stored on the floating gate, this represents a binary “0”. (These designations are arbitrary, and can be reversed so that the charged state represents binary “0” and the discharged state represents binary “1”.) That represents the programming “half” of how a floating gate memory cell operates. The other half is how to determine whether there is a charge stored on the floating gate—in other words, to “read” the memory cell. Generally, this is done by applying appropriate voltages to the source, drain and gate terminals, and determining how conductive the channel is. Some modes of operation for a floating gate memory cell are described hereinbelow.
Normally, the floating gate non-volatile memory (NVM) cell has only a single “charge-storing area”—namely, the conductive floating gate (FG) structure, and can therefore only store a single bit of information (binary “1” or binary “0”). More recently, using a technology referred to as “multi-level cell” (MLC), two or more bits can be stored in and read from the floating gate cell. MLC operation of memory cells is discussed in greater detail hereinbelow.
A Two-Bit (Dual Bit) Memory Cell
Another type of memory cell, called a “nitride, read only memory” (NROM) cell, has a charge-storage structure which is different from that of the floating gate memory cell and which permits charges to be stored in two separate charge-storage areas. Generally, the two separate charge storage areas are located within a non-conductive layer disposed between the gate and the underlying substrate, such as a layer of nitride formed in an oxide-nitride-oxide (ONO) stack underneath the gate. The non-conductive layer acts as a charge-trapping medium. Generally, electrical charges will stay where they are put in the charge-trapping medium, rather than being free to move around as in the example of the conductive floating gate of the floating gate memory cell. A first bit of binary information (binary “1” or binary “0”) can be stored in a first portion (such as the left-hand side) of the charge-trapping medium, and a second bit of binary information (binary “1” or binary “0”) can be stored in a second portion (such as the right-hand side) of the charge-trapping medium. An alternative viewpoint is that different charge concentrations can be considered for each bit of storage. Using MLC technology, as discussed in greater detail hereinbelow, at least two bits can be stored in and read from each of the two portions of the charge-trapping medium (for a total of 4 bits), similarly 3 bits or more than 4 bits may be identified.
FIG. 3 illustrates a basic NROM memory cell, which may be viewed as an FET with an “ONO” structure inserted between the gate and the substrate. (One might say that the ONO structure is “substituted” for the gate oxide of the FET.)
The ONO structure is a stack (or “sandwich”) of lower oxide 322, a charge-trapping material such as nitride 324, and an upper oxide 326. The ONO structure may have an overall thickness of approximately 10-25 nm, such as 18 nm, as follows:
the bottom oxide layer 322 may be from 3 to 6 nm, for example 4 nm thick;
the middle nitride layer 324 may be from 3 to 8 nm, for example 4 nm thick; and
the top oxide layer 326 may be from 5 to 15 nm, for example 10 nm thick.
The NROM memory cell has two spaced apart diffusions 314 and 316 (which can function as source and drain, as discussed hereinbelow), and a channel region 320 defined in the substrate between the two diffusion regions 314 and 316.
The charge-trapping material 324 is non-conductive, and therefore, although electrical charges can be stored in the charge-trapping material, they are not free to move around, they will generally stay where they are stored. Nitride is a suitable charge-trapping material. Charge trapping materials other than nitride may also be suitable for use as the charge-trapping medium. One such material is silicon dioxide with buried polysilicon islands. A layer (324) of silicon dioxide with polysilicon islands would be sandwiched between the two layers of oxide (322) and (326). Alternatively, the charge-trapping layer 324 may be constructed by implanting an impurity, such as arsenic, into a layer of silicon dioxide deposited on top of the bottom oxide 322.
The memory cell 300 is generally capable of storing two bits of data, a right bit in an area of the nitride layer 324 represented by the dashed circle 323 and a left bit in an area of the nitride layer 324 represented by the dashed circle 321. (MLC operation of the NROM, for storing a total of 4 bits of data is discussed hereinbelow.)
Each of the storage areas 321, 323 in the charge-trapping material 324 can exert a field effect on the channel region 320 between the source and the drain, in a manner similar to how a normal FET works, as described hereinabove (FIG. 2). some mechanisms for storing in the storage areas of the charge-trapping material, as well as removing charges from the storage areas of the charge-trapping material are described hereinbelow.
Generally, if a charge is stored in a given storage area of the charge-trapping material, this represents a binary “1”, and if no charge is stored in a given storage area of the charge-trapping material, this represents a binary “0”. (Again, these designations are arbitrary, and can be reversed to that the charged state represents binary “0” and the discharged state represents binary “1”.) That represents the programming “half” of how an NROM memory cell operates. The other half is how to determine whether there is a charge stored in a given storage area of the charge-trapping material—in other words, to “read” the memory cell. Generally, this is done by applying appropriate voltages to the diffusion regions (functioning as source and drain) and gate terminals, and determining how conductive the channel is. Some modes of operation for an NROM memory cell are described hereinbelow.
Generally, one feature of NROM cells is that rather than performing “symmetrical” programming and reading, NROM cells are beneficially programmed and read “asymmetrically”, which means that programming and reading occur in opposite directions. The arrows labeled in FIG. 3 are arranged to illustrate this point. Programming may be performed in what is termed the “forward” direction and reading may be performed in what is termed the “opposite” or “reverse” direction. Some programming and reading modes of operation for memory cells are described hereinbelow.
Memory Cell Modes of Operation and Injection Mechanisms
A memory cell's state may be defined and determined by what is called its threshold voltage (Vt) which determines a threshold level for the gate voltage required to establish the “channel” between the source and the drain—in other words, for the memory cell to begin to conduct current. A memory cell's threshold voltage level is directly related to the amount of charge (the number of electrons) stored in the charge storage region (floating gate, or ONO layer) of the cell—generally, more electrons stored means a higher threshold voltage Vt. Typically, for a given structure of a memory cell, the gate voltage that provides 1 pA (picoAmpere) of channel current is termed the threshold voltage (Vt).
The structure and general operation of two types of memory cells—floating gate and NROM—have been described hereinabove, with reference to FIG. 2 (floating gate) and FIG. 3 (NROM). Fundamentally, these two types of memory cells have in common with one another that they both operate very generally as a field effect transistor (FET)—namely, having two spaced-apart diffusion regions (functioning as source and drain) and a gate (for controlling the flow of electrons (current) through the channel region between the two diffusion areas, with the modification that they both have a charge storage structures under the gate.
The floating gate (FG) memory cell has a conductive layer between the gate and the channel region and, since the floating gate is a conductor, electrical charges stored in the floating gate are free to move around within the floating gate.
The NROM memory cell has a non-conductive layer (such as nitride) which can store charge in distinct areas and, since the non-conductive layer is not a conductor, the charges stored in the non-conductive layer are not free to move around, but rather tend to stay more-or less where they have been stored in a charge-storage region of the non-conductive layer, typically in a first region near one of the two diffusion regions, and in a second region adjacent the other of the two diffusion regions. These two diffusion regions are typically referred to as “left” and “right”, and the corresponding two charge-storage regions in the non-conductive layer are typically similarly referred to as “left” and “right”.
The concept of storing charges in the charge-storage structures (floating gate of a floating gate memory cell, or non-conductive layer of an NROM memory cell) has been discussed. The charges can also be removed from the charge-storage structure. Generally, the process of moving of charges into or out of the charge-storage structure is referred to as “injection”, and there are a number of known injection mechanisms.
In a general sense, “electrons” are (or behave as) negative charges and “holes” are (or behave as) positive charges. As we have heard many times in many contexts, “opposites attract”. For example, the north pole of a magnet attracts the south pole of another magnet, and is repelled by the north pole of another magnet. Generally, the same principle applies with electrical charges. An electron (negative charge) will be attracted by a positive electrical charge (or voltage) and will be repelled by a negative electrical charge (or voltage), and a hole (positive charge) will be attracted by a negative electrical charge (or voltage) and will be repelled by a positive electrical charge (or voltage).
The charge-storage structures of both the floating gate (FIG. 2) or NROM (FIG. 3) memory cells are separated from the channel region between the two diffusion regions by an insulating material, typically silicon dioxide (see tunnel oxide, FIG. 2; See lower oxide 322, FIG. 3). Therefore, in a general sense, the purpose of an injection mechanism is to move charges (electrons or holes) across the insulating material separating the channel region from the charge-storage structure of the memory cell.
The broad purpose of semiconductor memory is to store (“program”) data—many binary “1”s and “0”s—and allow access to (“read”) the stored data. And the broad purpose of a single memory cell is to store individual bits of the data. Single-level (SLC) floating gate memory cells can store one bit of data. Single-level (SLC) NROM memory cells can store two bits of data. Multi-level (MLC) floating gate memory cells can store two bits of data. Multi-level (MLC) NROM memory cells can store four bits of data. MLC operation of memory cells is discussed in greater detail hereinbelow.
Data is stored in and retrieved from memory cells in what is termed “modes of operation”. Generally, there are four modes of operation: “erase”, “program”, “write” and “read”. The first three modes (erase, program, write) relate to storing data. And, for purposes of this discussion, the write mode typically is simply a combination of erase and program. The read mode refers to accessing stored data.
The principle modes of operation discussed in this disclosure are program and erase. Both floating gate and NROM will be discussed.
Generally, to store a charge in the floating gate of a floating gate memory cell, the control gate's voltage may be raised with the source and drain grounded (or with the drain at a raised voltage), so that electrons tunnel through the tunnel oxide from the channel region to the floating gate, by a process known as CHE (channel hot electron) injection. To remove the charge (electrons) from the floating gate, the source's voltage may be raised with the gate grounded (or at a negative potential), and the electrons tunnel from the floating gate back to the substrate, by a process known as F-N (Fowler-Nordheim) tunneling (also abbreviated “FNT”).
FIG. 2A illustrates a technique for programming a floating gate memory cell, using channel hot electron (CHE) injection to put charges (inject electrons) into the floating gate. The floating gate memory cell generally comprises a channel region between a source region and a drain region, and a floating gate disposed between the channel region and the control gate (compare FIG. 2). This figure illustrates various voltages which may be applied to the source (Vs), to the gate (Vg) and to the drain (Vd), from external sources and/or connections (not shown). Generally, there is no “connection” to the floating gate.
Generally, in order implement CHE injection of electrons into the floating gate, the source is grounded, the drain is set to zero or to a positive voltage (which will “attract” electrons from the source, laterally across the channel region), and the gate is also set to a positive voltage (which will “attract” electrons vertically through the tunnel oxide, into the floating gate). As electrons flow through the channel from source to drain, some of the electrons will make their way through the tunnel oxide and become stored on the floating gate. This injection of electrons into the floating gate increases the threshold voltage of the memory cell. The shift (increase) in threshold voltage can be on the order of 3 or more volts. The threshold voltage (Vt) of the memory cell can later be measured, or “read”.
FIG. 2B illustrates a technique for erasing a floating gate memory cell, using a mechanism which is called “Fowler-Nordheim Tunneling”, abbreviated as “F-N tunneling”, or “FN tunneling”, or simply “FNT”.
Generally, whereas CHE injection was used (described hereinabove), in programming, to inject electrons into the floating gate, F-N tunneling (FNT) is used, in the erase operation, to remove electrons from the floating gate.
Generally, in order implement F-N tunneling of removing electrons from the floating gate, both the source and the drain are set to a positive voltage (to “attract” electrons through the tunnel oxide from the floating gate into the substrate), and the gate is set to a negative voltage (to “repel” electrons through the tunnel oxide from the floating gate into the substrate). This removal of electrons from the floating gate decreases the “threshold voltage” of the memory cell.
Generally, during programming, the threshold voltages of individual memory cells or (in the case of NROM) two charge-storage regions of a single memory cell) are individually manipulated to represent the data desired to be stored. In contrast thereto, generally, during erase, it is normally acceptable to simply decrease the threshold voltages of a great many memory cells, all at once, such as all of the memory cells in a sector or block of a memory array.
Typically, to inhibit erase of selected memory cells, an “inhibit” signal, such as a positive voltage (which will not “repel” the electrons) may be applied to the gates of the selected memory cells. In a common array architecture, the gates of several memory cells are typically all connected to a common word line (of many such word lines) in the array. Array architecture is discussed in greater detail hereinbelow.
Regarding “reading” the contents of a memory cell, no “injection mechanism” is used. The conventional technique of reading conductive floating gate memory cells is to apply appropriate “read voltages” to the gate and drain and to ground the source. This is similar to the method of programming with the difference being that lower level voltages are applied during reading than during programming.
Since the floating gate is conductive, the trapped charge is distributed evenly throughout the entire floating conductor. In a programmed device, the threshold is therefore high for the entire channel and the process of reading becomes symmetrical.
It makes no difference whether voltage is applied to the drain and the source is grounded or vice versa.
The following table presents exemplary conditions for programming, erasing and reading a floating gate memory cell.
TABLE 1Exemplary Floating Gate ConditionsVsVgVdVbtimeErase (FN)>=0−8-10v>=06-8 v100msProgram (CHE)gnd8-10v4-5 v??1μsRead0 v5v 1 v
FIGS. 3A and 3B illustrate a technique for programming an NROM memory cell, using channel hot electron (CHE) injection to inject electrons into the charge storage areas 321 and 323. As shown in FIG. 3A, the NROM memory cell comprises a channel region between two spaced-apart diffusion regions (left and right), and an ONO stack (322, 324, 326) between the channel region and the gate (328). (Compare FIG. 3.)
Generally, NROM memory cells may be programmed similarly to floating gate memory cells, using a channel hot electron (CHE) injection mechanism. Voltages are applied to the gate and drain creating vertical and lateral electrical fields which accelerate electrons from the source along the length of the channel. As the electrons move along the channel some of them gain sufficient energy to jump over the potential barrier of the bottom silicon dioxide layer 322 (of the ONO layer) and become trapped in the silicon nitride (charge trapping) layer 324 (of the ONO layer).
The NROM cell can store charges in two separate portions 321 and 323 of the charge-trapping layer 324. For purposes of this portion of the discussion, the left region 321 stores a left bit, and the right region 323 stores a right bit. Depending on which bit is desired to be programmed, the left and right diffusion regions 314 and 316 can act as source and drain, or as drain and source. The gate always functions as the gate.
FIG. 3A illustrates CHE programming of the right bit. In this example, electron trapping occurs in a region near the diffusion region 316 acting as a drain, as indicated by the dashed circle 323. Electrons are trapped in the portion 323 of nitride layer 324 near but above and self-aligned with the drain region 316 because the electric fields are the strongest there. Thus, the electrons have a maximum probability of being sufficiently energized to jump the potential barrier of the oxide layer 322 and become trapped in the nitride layer 324 near the drain 316.
FIG. 3B illustrates CHE programming of the left bit. For the left bit, the situation is reversed from programming of the right bit. In simple terms, the left diffusion area 314 functions as the drain and the right diffusion area 316 functions as the source, and electrons are sufficiently energized to jump the potential barrier of the oxide layer 322 and become trapped in the nitride layer 324 near the drain 314.
Generally, NROM memory cells may be erased using a technique called hot hole injection (HHI), or tunnel enhanced hot hole (TEHH) injection. For example, to erase an NROM memory cell, the source voltage can be set to a positive voltage such as +5 v, the gate voltage can be set to a negative voltage such as −7 v, and the drain voltage may be set to a positive voltage such as +2 volts (less than the source voltage) or may be left floating or disconnected.
Using HHI, holes (the “counterpart” of electrons) can be selectively inserted into the left portion 321 of the nitride layer 324 and into the right portion 323 of the nitride layer 324, in a controlled manner. Generally, holes which are injected cancel out electrons which are trapped (stored) in the left and right portions of nitride layer on a one-to-one basis (one hole “cancels out” one electron).
FIG. 3C illustrates erasing the right bit. In this example, hole injection occurs in a region near the diffusion region 316 acting as a drain, as indicated by the dashed circle 323. Holes are injected in the portion 323 of nitride layer 324 near but above and self-aligned with the drain region 316 because the electric fields are the strongest there. Thus, the holes have a maximum probability of being sufficiently energized to jump the potential barrier of the oxide layer 322 and become injected into the nitride layer 324 near the drain 316.
FIG. 3D illustrates HHI erasing of the left bit. For the left bit, the situation is reversed from erasing of the right bit. In simple terms, the left diffusion area 314 functions as the drain and the right diffusion area 316 functions as the source, and holes are sufficiently energized to jump the potential barrier of the oxide layer 322 and become injected into in the nitride layer 324 near the drain 314.
Reading an NROM memory cell may involve applying voltages to the terminals of the memory cell comparable to those used to read a floating gate memory cell, but reading may be performed in a direction opposite to that of programming. Generally, rather than performing “symmetrical” programming and reading (as is the case with the floating gate memory cell, described hereinabove), the NROM memory cell is usually programmed and read “asymmetrically”, meaning that programming and reading occur in opposite directions. This is illustrated by the arrows in FIG. 3. Programming is performed in what is termed the forward direction and reading is performed in what is termed the opposite or reverse direction. For example, generally, to program the right storage area 323, electrons flow from left (source) to right (drain). To read the right storage area 323 (in other words, to read the right “bit”), voltages are applied to cause electrons to flow from right to left, in the opposite or reverse direction. For example, generally, to program the left storage area 321, electrons flow from right (source) to left (drain). To read the left storage area 321 (in other words, to read the left “bit”), voltages are applied to cause electrons to flow from left to right, in the opposite or reverse direction. See, for example, commonly-owned U.S. Pat. No. 6,768,165.
The following table presents exemplary conditions for programming, erasing and reading an NROM memory cell.
TABLE 2Exemplary NROM Gate ConditionsVsVgVdVbtimeProgram (CHE)+5 v8-10v0 v1 μsErase (HHI)  5 v−7v2 v??Read05v1.3 v  “Vs” refers to the left diffusion area, and “Vd” refers to the right diffusion area, for the operations of programming, erasing and reading the right side bit of an NROM memory cell.
The operations of program and erase are typically performed using pulses, each pulse partially moving (nudging) the memory cell towards the desired Vt, followed by verify (a quick read, to see if the desired Vt has been reached), until the desired Vt has been attained. Typically, conditions are established so that only a few (for example, 3-5) pulses are required to program or erase each cell.
Exemplary operating modes for memory cells, using the mechanism of CHE injection for programming a memory cell, and the mechanisms of FNT and HHI for erasing a memory cell have been described, hereinabove. Other and additional mechanisms are known for performing the modes of operation.
Other Erase and Program Operations for NROM
The article “A Single-Sided PHINES SONOS Memory Featuring High-Speed and Low-Power Applications”, IEEE Electron Device Letters, Vol. 27, No. 2, February 2006, incorporated in its entirety by reference herein, discloses erase and program operations for a memory cell having ONO thicknesses of 5 nm (bottom oxide), 7 nm (nitride) and 9 nm (top oxide), and being operated as a one-bit/cell device used in a virtual ground array. As noted in the article, CHE programming has low programming efficiency and causes high power consumption.
First the memory cell is erased to a high Vt level by a negative FN electron tunneling operation wherein, a high negative voltage is applied to the gate of the memory cell in a single terminal (1-terminal) operation (the source and drain voltages are left at 0 volts), and electrons tunnel into the nitride charge storage area. See FIG. 3E.
The erase conditions set forth in the article are Vs=0 v; Vg=−20 v; Vd=0 v; Vb=0 v. See Table 3, below. An upward Vt shift of approximately 3 volts (from 0 to 3 volts) can be achieved in approximately 1 millisecond (1 ms).
The negative FNT erase operation being a single terminal operation, the entire charge storage area of the memory cell is populated with electrons. This is shown in FIG. 3E by the nitride layer being filled with electrons (represented as dashes in circles). And, when multiple memory cells are tied to a common word line, all of the memory cells tied to that word line will be erased. Erase is generally a “bulk”, indiscriminate operation, intended to bring a plurality of memory cells to a predefined state (either logic “1” or logic “0”), at once.
Then, for programming the cell, band-to-band hot-holes (HHs) are injected to both the left side and the right side to decrease the cell Vt, and a programming speed as fast as 20 μs can be achieved. See FIGS. 3F, 3G.
Hot Hole Injection (HHI) is a two-terminal operation, so only one side can be done at a time. In FIG. 3F, holes (represented as plus signs in circles) fill the left side of the nitride layer, “canceling out” a like number of electrons. In FIG. 3G, holes (represented as plus signs in circles) fill the right side of the nitride layer, “canceling out” a like number of electrons. (In the diagrams of FIGS. 3F and 3G, the electrons are illustrated as being in the top oxide layer not because they are there—they aren't—but rather because they simply do not fit in the illustration within the nitride layer—where they are. The electrons are in the nitride layer, as shown in FIG. 3E.)
The program conditions set forth in the article are Vs=0 v; Vg=−10 v; Vd=0 v; Vb=0 v. See Table 3, below. A downward Vt shift of approximately 3 volts (from 0 to −3 volts) can be achieved in approximately 20 microseconds (20 μs).
Since the device channel stays off during both programming and erase operation, the power consumption is very low. The article claims to achieve a high programming throughput of 10 MB/s with low power (program current<10 nA/cell).
However, it should be realized that the memory cell is being used as a one-bit/cell device, rather than as a two-bits/cell device, as in NROM. (The article appears to claim that two-bits/cell can be achieved by MLC operation).
TABLE 3Exemplary NROM Gate ConditionsVsVgVdVbtimeErase (FNT)0 v−20 v0 v0 v1msProgram (HHI)5 v−10 v5 v0 v20μs
The erase and program operations may typically be performed using a few cycles of pulses followed by verify (read).
There are, of course, many nuances to each of the operations of memory cells discussed hereinabove. For example, repeated erasing of a memory cell can result in lowering the threshold voltage beyond zero, becoming negative, a condition known as “over-erase”. And, particularly with regard to programming NROM memory cells in an array of many NROM cells, programming one bit of a given cell may disturb the state of a neighboring cell. These issues, as they may become relevant and pertinent, are discussed hereinbelow.
For example, U.S. Patent Publication No. US2005/0078527, incorporated in its entirety by reference herein, discloses a method of over-erase protection in a NROM device by performing a F-N tunneling program to provide additional electrons to the nitride charge-trapping layer (of a NROM device having an ONO layer) to increase the threshold voltage before a CHE program cycle or before a HHI erase cycle. The FNT step may be applied to individual NROM devices, or to a plurality of NROM devices.
Programming is typically performed in increments, with pulses of voltage—after each pulse, a verify operation occurs in which the threshold voltage level of the cell is measured (read). The general idea is to “nudge” the threshold voltage to the desired level, rather than over-shooting (over programming) or under-shooting (under programming) the desired level. With appropriate control mechanisms, only a few pulses (nudges) are required. A similar concept of cycles of pulse followed by verify until a desired Vt has been attained may sometimes be used during the erase operation, to avoid under-erase or over-erase. See, for example, commonly-owned U.S. Pat. Nos. 6,292,394; 6,396,741; 6,490,204; 6,552,387; 6,636,440; and 6,643,181.
Multi-Level Cell (MLC) Operation of Memory Cells
Mention has been made, hereinabove, of single level cell (SLC) and multi-level cell (MLC) operation, and it shall be described only briefly in this disclosure.
Theoretically, in order to determine the programmed state of a memory cell, only one voltage threshold is needed—either the threshold voltage (Vt) of the cell is below the threshold, or over the threshold (Vth). However, this simplistic approach can lead to ambiguities and false readings. This is in part due to the fact that the charges (such as electrons) cannot be stored (in the floating gate, or in the NROM storage region) with absolute precision, and is in part due to the fact that sometimes electrons disappear from the storage region.
Therefore, in practice, to store one bit of data, two voltage levels are needed. If the sensed threshold voltage is below the lower of the two voltage levels, that is classified as a “0”, and if the sensed threshold voltage is above the higher of the two voltage levels, that is classified as a “1”.
Memory cell technology has been developed wherein memory cells can hold two or more bits of data, instead of just one each, in the storage region. The trick is to take advantage of the analog nature of the charge stored in the memory cell and allow it to charge to several different voltage levels. Each voltage range to which the floating gate can charge can then be assigned its own digital code. This is generally referred to as “Multi-Level Cell (MLC)” technology.
Injection Mechanisms, Generally
A number of “injection mechanisms” have been described hereinabove, in the context of modes of operating memory cells, such as CHE, FNT, HHI.
Generally, an injection mechanism includes any mechanism that causes electrons to be inserted into the storage area (floating gate) of a floating gate memory cell, or into the left or right storage area (in the ONO layer) of an NROM memory cell, such as CHE and FNT.
Generally, the more electrons you can insert into the storage area, the higher the threshold voltage of the memory cell will be. For a single level cell (SLC), a threshold voltage Vt above a predefined level Vth may be designated to represent either a logic “1” or a logic “0”, and a threshold voltage Vt below the predefined level Vth may be designated to represent either a logic “0” or a logic “1”.
Generally, an injection mechanism also includes any mechanism that causes electrons to be removed (extracted) from the storage area (floating gate or ONO region) of a memory cell. Generally, the threshold voltage of the memory cell can be lowered by removing previously-inserted electrons from the storage area.
Another way to lower the threshold voltage of the memory cell is to “cancel out” the electrical charge effect of previously-inserted electrons in the storage area, and this may be accomplished by inserting holes into the storage area, such as with HHI. Generally, one hole will “cancel out” one electron.
These injection mechanisms are generally well known. Although the mechanisms may largely be dominated by the various voltages applied to the source (Vs), gate (Vg) and drain (Vd) of the memory cell, as well as to the substrate (Vb), factors such as materials used in various elements of the memory cell, as well as geometry and dimensions of the elements of the memory cell can also significantly affect the injection mechanism.
Memory Array Architecture, Generally
Memory arrays are well known, and comprise a plurality (many, including many millions) of memory cells organized (including physically arranged) in rows (usually represented in drawings as going across the page, horizontally, from left-to-right) and columns (usually represented in drawings as going up and down the page, from top-to-bottom).
As discussed hereinabove, each memory cell comprises a first diffusion (functioning as source or drain), a second diffusion (functioning as drain or source) and a gate, each of which has to receive voltage in order for the cell to be operated, as discussed hereinabove. Generally, the first diffusions (usually designated “source”) of a plurality of memory cells are connected to a first bit line which may be designated “BL(n)”, and second diffusions (usually designated “drain”) of the plurality of memory cells are connected to a second bit line which may be designated “BL(n+1)”. Typically, the gates of a plurality of memory cells are connected to common word lines (WL).
FIG. 4 illustrates an array of NROM memory cells (labeled “a” through “i”) connected to a number of word lines (WL) and bit lines (BL). For example, the memory cell “e” has its gate connected to WL(n), its source (left hand diffusion) is connected to BL(n), and its drain (right hand diffusion) is connected to BL(n+1). The nine memory cells illustrated in FIG. 4 are exemplary of many millions of memory cells that may be resident on a single chip.
Notice, for example that the gates of the memory cells “e” and “f” (to the right of “e”) are both connected to the same word line WL(n). (The gate of the memory cell “d” to the left of “e” is also connected to the same word line WL(n).) Notice also that the right hand terminal (diffusion) of memory cell “e” is connected to the same bit line BL(n+1) as the left-hand terminal (diffusion) of the neighboring memory cell “f”. In this example, the memory cells “e” and “f” have two of their three terminals connected together.
This means that if you want to perform an operation (such as program, erase or read) on one cell something, there could be an effect on a neighboring cell, since two of the three terminals of adjacent memory cells are connected together (gate connected to gate, and one of left/right diffusions connected to one of right/left diffusions for the neighboring cell.)
Generally, as discussed in greater detail hereinbelow, it may be relevant whether a given operation (such as program, erase or read) is performed as a “I-terminal” operation (such as FNT erase to high Vt, discussed hereinabove), as a “2-terminal” operation (for example, applying a non-zero voltage to the gate and to one of the source or drain diffusions) or as a “3-terminal” operation (for example, applying a non-zero voltage to the gate and to both of the source or drain diffusions).
Generally, when programming or erasing a cell, one or more of the neighboring cells may be affected by the programming/erasing operation, causing thereto a possible change in their threshold voltage. This unwanted change in threshold voltage of unselected cells is known as a “disturb”. A similar effect (disturb) may occurs during a read operation. However, due to the relative weakness of the applied voltage levels, the effect during read is significantly smaller.
The situation of neighboring memory cells sharing the same connection—the gates of neighboring memory cells being connected to the same word line, the source (for example, right hand diffusion) of one cell being connected to the drain (for example left hand diffusion) of the neighboring cell—is even more dramatically evident in what is called “virtual ground architecture” wherein two neighboring cells actually share the same diffusion. In virtual ground array architectures, the drain of one memory cell may actually be the same diffusion which is acting as the source for its neighboring cell. Examples of virtual ground array architecture may be found in U.S. Pat. Nos. 5,650,959; 6,130,452; and 6,175,519, incorporated in their entirety by reference herein.
A memory array may also include isolation zones (not shown). Isolation zones segregate one group of memory cells from a neighboring group of memory cells, for example isolation zones can divide the array into slices of just one column or a plurality of columns. Examples of arrays having isolation zones may be found in commonly-owned U.S. Pat. No. 7,043,672, incorporated in its entirety by reference herein, and in coinmonly-owned U.S. Pat. No. 6,975,536.
A more complete description of NROM and similar ONO cells and devices, as well as processes for their development may be found at “Non Volatile Memory Technology”, 2005 published by Saifun Semiconductor and materials presented at and through http://siliconnexus.com, both incorporated by reference herein in their entirety.